Conformal, low profile, and wideband phased arrays have received increasing attention due to their potential to provide multiple functionalities over several octaves of frequency, using shared common apertures for various applications, such as radar, ultra-fast data-links, communications, RF sensing, and imaging. These arrays offer tremendous advantages, including multiple independently steerable beams, polarization flexibility, and high reliability.
To achieve ultra-wideband operation, numerous array antennas, such as tapered-slot-antennas or Vivaldi arrays, “bunny ear” arrays, long-slot arrays, and tightly-coupled arrays (TCA), have been proposed.
Since the radiating elements of an antenna in an antenna array, such as the TCA, may take symmetric forms, such as dipoles and bowties, wideband balanced feeding is often used to excite the antenna to attain an antisymmetric current distribution (i.e. with currents in the two dipole arms equal in amplitude and 180° out of phase). Unbalanced feeding will result in common mode excitation, impedance instability, and/or high cross-polarization, thereby significantly degrading the operational bandwidth.
In certain antenna arrays, such as the TCA, the high input resistance imposes another difficulty, namely to provide appropriate impedance matching with a conventional 50-Ω coaxial line. As a result, a balanced-to-unbalanced transformer, i.e., a balun, as well as an impedance transformer, is typically provided for each radiating element. The use of these transformers, however, can impose additional restrictions on the performance of the antenna array, such as the bandwidth, operational frequency, weight and profile, particularly at high operational frequencies, conformability, overall compactness and the additional relative high costs of these components.
In addition, such electrically fed phased arrays may be also subjected to scan blindness, a common phenomenon that occurs due to a large input mismatch at a certain scan angle when a spatial harmonic of the periodic array resonates with antenna feed circuitry. This may limit the scan range of the phased arrays.